Controllable electric body tissue stimulators



March 28, 1967 D. L. BOWERS 3,311,111

CONTROLLABLE ELECTRIC BODY TISSUE STIMULATORS Filed Aug. 11, 1964 INVENTOR DAVID L. BOWERS BYf g yw ATTORNEY n fi FIG. 6

United States Patent 3,311,111 CONTROLLABLE ELECTRIC BODY TISUE STIMULATORS David L. Bowers, Milwaukee, Wis, assignor to General Electric Company, a corporation of New York Filed Aug. 11, 1964, Ser. No. 388,758 1 Claim. (Cl. 128422) This invention pertains generally to devices for electrically stimulating tissue and organs in living bodies and it is particularly concerned with controlling the electric output circuits and characteristics of such devices from a position that is remote from the site of body implantation.

It is known that certain body tissues can be caused to exhibit peristalsis, dilation, contraction and other intrinsic functions by stimulating them with an electric current. This phenomena has many possible uses one of which is demonstrated -by the electronic cardiac Pacemaker. An electronic cardiac Pacemaker has a sefl-contained power supply and oscillator which is implanted in a body and produces electric pulses that are conducted to the heart muscle by suitable leads. The pulses are of. such frequency, duration, amplitude, and in some instances, phasing, as to fairly well simulate the natural electric acitvity of the heart in which case the heart is caused to beat at a fixed rate. A fixed rate stimulator, of course, does not take into account the fact that the activity of the controlled organ or tissue should be changed in accordance with varying body needs. Therefore, it is evident that some means for externally controlling the action of the stimulator is needed.

In one presently available implantable stimulator external control is obtained with a portable pulse source that is connected to an air core induction coil which may he placed over the implanted pulse source. There is another induction coil, acting as a secondary Winding, which is in series with a capacitor in the implanted device. The pulse rate of the external source is continuously adjustable within limits, and is set faster than the natural frequency of the implanted oscillator, causing the implanted oscillator to be slaved to the external triggering source This results in earlier discharge of the capacitor, and hence, an increased pulse rate from the implanted device.

An external control of the type just outlined facilitates adjusting the heart rate to the level of body activity that is contemplated. However, it also has disadvantages. Because of the loose magnetic coupling, it must necessarily produce pulses of high energy in order to account for losses. This causes the batteries that drive the external pulse source to discharge rather rapidly. In addition, the external source is not so convenient to carry or use because the external induction coil must be taped on the body surface for continual utilization. If the coil is removed from the body, the implanted stimulator has no memory and will drop back to its natural or minimum rate that is governed by its circuit parameters.

It has been discovered that stepless control of the opcrating characteristics of implanted stimulators is not imperative. For instance, in artificial cardiac Pacemakers, it is usually suificient if there is one rate setting of about sixty-five pulses per minute for low physical activity and another setting of about eighty-five pulses per minute for high physical activity. Other stimulators may have a different range and number of discrete settings.

Accordingly, it is a primary object of this invention to overcome the aforementioned disadvantages and to provide means for selecting discrete operation modes and characteristics of implantable devices externally of the body.

Other important objects of the present invention are as follows: to provide an external control for implanted stimulators that consumes no electric power, that is easy to carry and operate, that does not require permanent aitixation to the body, that has a self-contained memory for the last setting of its characteristics, that allows simple mode selection, that is highly miniaturized, and that is reliable, rugged, and fail sate.

Achievement of the foregoing and other more specific objects will appear from time to time throughout the course of the ensuing specification.

In general terms, a preferred embodiment of the present invention may be characterized as an implantable stimuulator which includes bi-stable switches, such as a biased magnetic reed type, that may be actuated by an auxiliary magnetic field applied in an appropriate region externally of the body. Adjacent each reed switch is a bias magnet that holds the reed switch closed after it is urged closed by the auxiliary magnetic field but which bias magnet is of insufficient strength to act on the reed switch in the absence of another magnetic field. By this means one or more reed switches may be switched to change the operational mode of the stimulator and yet the switching states will be memorized and retained until they are supervened by another application of an external magnetic field. In

its broadest concept, the present invention is not confined to the use of magnetic reed switches but extends to switching arrangements that have two stable states each of which may be magnetically selected.

The bi-stable switches may be used to switch in and out resistors or capacitors to control either the charging or discharging of an RC circuit and thereby control the pulse rate of cardiac stimulator, for instance, or they may be adapted to select parameters which alter voltage, current, and duration, or total energy of electric pulses. In addition, the bi-stable switches may be actuated magnetically to select different output paths or leads.

The invention is further characterized as being essentially a logic switching rather than power switching device. That is, the bi-stable switches are primarily involved in changing operational mode rather than switching power circuits that would raise concern about current interruption and conducting capacity of the switches.

Illustrative embodiments of the invention will now be described in greater detail in conjunction with the drawing in which:

FIGURE 1 is a perspective view of an implantable stimulator associated with a permanent magnet for actuating the same;

FIGURE 2 is a plan view of a bi-stable reed switch associated with its bias magnet and a magnet for actuating the switch remotely;

FIGURE 3 shows a circuit diagram for a self-contained implantable stimulator that incorporates the features of the invention and is associated with an external control of a known type;

FIGURE 4 shows a modification of a part of the circuit which is depicted in FIGURE 3;

FIGURE 5 is an elevation View of another form of implantable stimulator which incorporates features of the invention; and

FIGURE 6 shows a group of cards A-D on which there are afiixed permanent magnets that facilitate programming of implantable stimulators in accordance with the invention.

FIGURE 1 shows a stimulator the container of which is designated ;by the reference numeral 10. The container may constitute an encapsulation for electronic components such as those shown in FIGURE 3. Usually these components are assembled on a circuit board, not shown, and encapsulated in epoxy resin which is covered with body compatible silicone rubber. The container 1% may be surgically implanted in a body with the output leads 12 and 13 connected to any organ that it is desired to stimulate. Of course, these leads are insulated over most of their length and terminate in bared wire ends 14 which may serve as electrodes or they may be connected with any suitable electrodes, not shown, in

order to establish electrical connection with tissue.

In FIGURE 1, a bi-stable switch such as a read switch 15 'is shown adjacent its bias magnet 16. The circuitry with which switch 15 is associated has been omitted from FIGURE '1 for the sake of clarity. An external permanent magnet 17 is shown in a position where, depending upon its polarity, it might reinforce or nullify "the action of bias magnet 16 to actuate switch 15. The

results achieved by placing auxiliary magnet 17 on or near the surface of the body in approximate alignment with stimulator 10 to actuate the switch will be discussed more fully hereinafter.

Referring now to FIGURE 3 there is illustrated a circuit diagram of a suitable relaxation oscillator 11 which 'has output terminals 18 and 19. Leads 12 and 13 run from these terminals to a simulated RC load which is set off with broken lines 20 and includes a capacitive element 21 and a resistive element 22. If the approximate load equivalent of a human heart is represented by the elements in box 20, capacitor 21 would have an equivalent value of approximately. 20 microfarads and resistance I 22 would have an equivalent value of approximaetly 300 ohms.

{leads 12, 13 emerge are sealed in insulating encapsulation container 10 which was mentioned earlier.

The oscillator may be variously designed but the preferred form used here is similar to one that is described in a co-pending application in the name of Heinz Raillard, Ser. No. 101,583, entitled Transistorized Negative Resistance Networks, filed Apr. 7, 1961, now Patent No. 3,144,620, dated Aug. 11, 1964, and assigned to the assignee' of the instant invention. This relaxation oscillator comprises a pair of complementary NPN and PNP transistors 23, 24, respectively. The circuit is supplied with direct current from a pair of batteries 26 and 27 which are preferably long life mercury cells.

As shown, the positive terminal of battery 26 is connected through a resistor 28 to the emitter 29 of PNPtransistor 24. Through a diode 30 and a resistor 31 a positive voltage fromthe same battery 26 is also supplied to a common conductor 32 that connects the base 33 of the PNP transistor with the collector 34 of the NPN transistor. A common line 37 connects the collector 38 of transistor :24 to the base 39 of transistor 23. The positive terminal of'battery 27 is connected to a biasing resistor that connects to the junction of the base electrode of transistor 23 and the collector electrode of transistor 24, and to the negative terminal of battery 26. Batteries 26 and 27 serve to charge a capacitor 40 through transistors 23 and 24. A stabilizing circuit including the series connection of 'a resistor 31 and a diode 30 is connected between the positive terminal of battery 26 and the base electrode of transistor 24, diode 39 being polarized in the forward direction with respect to the batteries.

Also in the circuit are some serially connected resistors 41, 42, and 43 which constitute a discharge circuit for the series capacitances 21 and 49. When the total resistance of these three resistors is high, the pulse rate appearing at terminals 18 and 19 will be low. On the other hand, when the total resistance is low, that is, when some of the resistance is shorted out by reed switch 15, the pulse rate will be increased due to the shorter time constant.

'It' is optional to have an inductor 44 in the circuit in parallel with a polarized diode 45. The purpose of inductor 44 is to allow continuous variation of the pulse rate by magnetically coupling it with another inductor 46 that is supplied by an external variable pulse source 47. This external control including inductors 44 and 46, diode 45 and pulse source 47 is not part of the instant invention and is more fully explained in a co-pending application of H. W. Abbott and H. Raillard, entitled, Low Duty Cycle-High Efiiciency Transistor Pulse Generator and Circuit Application, filed July 6, 1961, Ser. No. 122,135, new Patent No. 3,185,940, dated May 25, 1965, and assigned to the assignee of the instant invention. For present purposes, it is sufficient to say that the pulse rate appearing at terminals 18 and 19 can be made higher than the natural frequency of oscillator circuit 11 by coupling ulses from source 47 with inductors 44 and 4-6. As intimated earlier, this causes capacitor 40 to charge sooner to thereby cut-off the transistors and initiate an earlier pulse discharge through resistors 41, 42, and 43 which causes a corresponding pulse to appear across load 20.

The description may proceed on the assumption that some suitable oscillator lies to the left of junctions 48 and 49 in FIGURE 3 and that the pulse rate of such oscillator will be governed by the capacitive value of the series capacitances 21 and 40 taken in conjunction with a discharge circuit that includes resistors 41, 42, and 43. When these last three resistors are all in series between junctions 48 and 49,- the pulse rate will be the lowest. In a cardiac Pacemaker, the lowest rate is usually around 65 pulses per minute which corresponds with relatively low physical activity. In order to facilitate obtaining a higher rate, there is provided a reed switch 15 which is shown in FIGURE 3 in one of its stable states with its magnetically susceptible reed contacts 50 and 51 separated from each other to establish an open circuit condition in which case the combined resistance of resistors 41, 42, and 43 is in circuit. When reeds-5t and 51 of switch 15 are closed by an externally applied auxiliary magnetic force, the discharge path between junctions 48 and 49 includes only resistor 41 and the reed switch 15. Thus, resistors 42 and 43 are by-passed and the oscillator operates at a higher pulse rate.

As stated earlier, an important aspect of the invention resides in changing the operating characteristics or operational mode of a body implanted stimulator by actuating one or more bi-stable devices with an auxiliary magnetic force from a non-power consuming device such as a permanent magnet or an electro-magnet that may supply magnetic impulses. A further feature is that the bistable device is impressed with a memory and remains in whatever switching state to which it has been urged without power consumption until it is acted upon again by an external magnetic force which changes its states. In this illustrative example, the bi-stable device takes the form of reed switch 15 which co-acts with a bias magnet 16 and an externally applicable permanent magnet 17 which are shown in FIGURE 2. Attention is invited to this figure for the purpose of explaining the function of these elements. It will be seen that the reed switch 15 includes a pair of magnetically susceptible reeds 50 and 51 which by the spring action of 51 maintain themselves in a separated position unless urged together by an auxiliary magnetic force. In other words, reeds 5t and 51 are normally open circuited. As can be seen in FIGURES 1 and 3 as well as FIGURE 2, there is juxtaposed with reed switch 15 a bias magnet 16 which may be a bar of square or other cross-section. The bias magnet sets up a magnetic field along the path of broken lines 52 and 53 which magnetic path includes reeds 50 and 51. The strength of bias magnet 16 is insufiicient by itself to cause reed 51 to snap into contact relation with reed 50-, and hence, the reed switch remains open. Now, if an auxiliary magnet 17 is brought into proximity with the reed switch and if the corresponding upper and lower pole faces of magnets 16 and 17 are the same asshown, the field from magnet 17 will supplement that of bias magnet 16 and the reed switch will snap closed. Magnet 17 may then be removed from the vicinity of the reed switch but the latter will maintain its closed state under the influence of the magnetic attractive force from bias magnet 16. Those versed in the art will recognize that magnet 17 must have sufficient field strength to overcome the inherent selfopening characteristic of the reed switch in spite of an inch or more of body tissue intervening between the magnet and the switch. It is also important thatthe strength of magnet 17 be not so great as to induce a polarity change in bias magnet 16 when the two magnets are within the closest attainable distance from each other.

In FIGURE 3, with the reed switch 15 closed in accordance with the above discussion, only resistor 41 will remain in the circuit, resistors 42 and 43 being by-passed, in which case the stimulator would operate at a higher pulse rate. Under these circumstances, a patient would be ready for increased activity.

Reed switch 15 will remain in closed position until it is acted upon by again placing magnet 17 in proximity with it when the poles at opposite ends of magnets 16 and 17 are of unlike polarity. This amounts to turning magnet 17 end for end in FIGURE 2. In one design, magnet 17 is encapsulated in a white plastic body which is streamlined for comfortable storage in a patients purse or pocket between uses. This inauspicious object is imprinted on one side with the word active and on the other with the word normal so that the patient may verify that he is selecting the desired stimulator program by looking down on the magnet while bringing it toward his body. In another design, the external magnet 17 is a bar magnet that is enclosed in a tube, not shown, that looks like an automatic pencil and may be clipped in the patients pocket.

Of course, when opposite poles of the bias magnet 16 and movable magnet 17 are aligned-with each other, the stronger magnetic field of magnet 17 overcomes the bias magnet 16 in which case reed 51 snaps to its open circuit position and the stimulator reverts to its lower rate that prevails when resistors 41, 42, and 43 are all in circuit with the series capacitance of 40 and 21.

The logic switching described in the preceding paragraphs may be extended to switching programs including four or more steps as will now be explained in connection with FIGURES 4, 5, and 6.

FIGURE 4 may be considered as being broken away from oscillator 11 circuit in FIGURE 3 and modified between junction points 48 and 49 by the inclusion of an additional bias magnet 16' and a reed switch 15' which parallels an added resistor 54. In FIGURE 4 it will be seen that when series resistors 41, 54, 42, and are 43 are all in circuit with the reed switches 15 and 15 open, the stimulator will operate at its lowest of four rates. If we assume that resistor 54 is smaller than the series resistance 42 and 43, it will operate at its next higher rate when reed switch 15 is closed to by-pass resistor 54 in which case resistors 41, 42, and 43 remain in circuit. It will operate at its next higher rate when reed switch 15 is closed and reed switch 15' is open in which case resistors 41 and 54 will be in circuit. Its highest rate will be attained when both of the reed switches 15' and 15 are closed so as to place only resistor 41 in circuit as a discharge path for capacitances 40 and 21.

When multiple step logic switching functions are carried on as just described in connection with FIGURE 4, it is desirable to locate the bi-stable switches 15 and 15 in spaced relationship with respect to each other as illustrated in FIGURE 5. With this arrangement, a single external magnet may be used to actuate the one reed switch without interfering with operation of the other if such operation is not desired. In order to eliminate the need for the operator being careful of exactly where he locates the external magnet when changing the program of a stimulator that is equipped with multiple bi-stable switches, means are provided as shown in FIGURE 6 for conveniently programming the stimulator. In FIGURE 6 are shown a group of program cards 6A-6-D on which there are afiixed a pair of magnets 70 and 71 which have dots on them, such as dot 72, to indicate their north poles, for example. The dots are disposed toward index marks 73 and 74. The card of FIGURE 6-A may be marked with a numeral such as 65 which is indicative of one of the pulse rates that a patient may want to program. The individual auxiliary magnets 70 and 71, for example, serve the purpose of permanent magnet 17 as previously discussed. When a magnet such as 71 on the card of FIGURE 6-A is placed over a bias magnet 16 and its associated reed switch 15 in FIGURE 5 with like poles of the magnets 71 and 16 coinciding, the switch 15 will close. When the polarities of the magnet on the card and bias magnet are opposite when these magnets are superposed, the associated reed switch will open. For example, when the card of FIGURE 6-A is positioned on a patients body over an implanted stimulator such as shown in FIGURE 15, bar magnets 71, 70 will open reed swtiches 15 and 15', respectively, to bring about a pulse rate of 65 pulses per minute. If the patient wants to increase his heart rate for more vigorous physical activty, he may elect press card C against his body in proximity with the stimulator so as to bring about a heart beat rate of 95 pulses per minute in which case switch 15 would be closed and switch 15 would be open. To obtain pulses per minute, the card of FIGURE 6 8 would be placed over the stimulator in FIGURE 5, to cause switch 15 to open and switch 15' to close. Without going back to any lower rate, the patient may increase his heart rate another step by placing the card of FIGURE 6-D on or in proximity with his body in reasonable alignment with the implanted device 10 of FIGURE 5 to bring about the highest rate. Relating back to FIGURE 4, this is tantamount to closing both reed switches 15 and 15 in which case resistors 54, 42, and 53 are shunted and only resistor 41 is between junctions 48 and 49 in circuit with capacitors 40 and 21.

The program cards shown in FIGURE 6 are only illustrative of how the bi-stable switches may be sequenced and how difierent output characteristics of a stimulator may be achieved without requiring exercise of critical judgment by the user. For instance, it is not necessary that magnets 70 and 71 be placed in fixed positions on individual cards. One may provide a single card or mounting, not shown, which rotates the actuating magnets into polarized positions that correspond with the desired rate or other characteristic of the stimulator. Also electromagnets could be used and their polarity changed by switching current direction through the coils. Another alternative, not shown, is to allow the patient to locate manually the actuating magnets on a card on which there are appropriate code and index marks for making sure that the desired program is established.

Mode selection discussed above deals primarily with switching series resistors to illustrate the principles of the invention. However, those versed in the art will apptreciate that mode selection might involve switching capacitors, resistors or other circuit elements in series and parallel circuits. In more complex stimulators, the logic switching with bi-stable devices may be extended to tapping supply batteries for different voltages, reversing polarities, changing input and output paths, and so forth.

The above described logic switching system is extremely reliable and easy for a patient to employ. Extrapolation of test data indicates that reed switches may operate up in the hundreds of millions of times with a low probability of failure. Many early failures may be due to production defects which appear in the first 100,000 operations so all of the switches used in stimulators, especially cardiac Pacemakers, are operated at least that many times before they are selected for installation. It

is calculated that a patient may over a live year period reprogram a stimulator around 10,000 times so that a lifetime of operational capability will ordinarily remain even though the life of the batteries that drive the stimulator may have expired.

Because the bi-stable switches are primarily logic switches rather than power handling devices, and because of the low available power, there is no chance that the reed contacts 50 and 51 will weld together. If a highly improbable failure of the reed switch' occurred, it would result in the switch remaining in a fail-safe or open circuit condition which would bring about operation of the stimulator at a low rate or energy level. Ordinarily this will reduce body efficiency but will not bring about a catastrophe during the intervalwhen professional aid may be solicited. In models that are now available, highly stable Alnico magnet material has been used for both the bias magnets 16 and the portable magnet 17. These magnets have been foundto have adequate retentivity and to be immune from being repolarized or demagnetized by the kinds of extraneous magnetic fields that are ordinarily encountered by a patient in which a stimulator has been implanted. Nevertheless, the even more stable ceramic magnet material may be preferred for the portable magnet 17 and especially for the bias magnet 16. And, of course, any portable magnet problems can be obviated by use of an electro-magnet, not shown, for the permanent magnets 17, 70 or 71. A suitable electro-magnet system may include a battery and a storage capacitor which discharges into a coil when a pushbutton circuit is closed to provide a single pulse for pulses for operating the various bi-stable switches. The electro-magnet package is not depicted because it can be easily fabricated by one versed in the art after having received the suggestion herein presented.

In summary, there has been described a body implantable stimulator that features remote selection of its operating characteristics, that includes a self-contained memory for the last setting .to which it has been subjected, that has simple'rnode selection, that is highly miniaturized and reliable and consumes no electric power. Although the invention has been described primarily with respect to changing the output pulse rate of a stimulator, it will now be perceived by those versed in the art that the same inventive concepts may be utilized to bring about various operational modes. Hence, the described embodiments are to be considered illustrative rather than limiting for the invention may be variously embodied and is to be limited in scope only by construction of the claim which follows.

It is claimed:

A body implantable artificial cardiac Pacemaker comprising:

(a) a body compatible implantable insulating encapsulation container,

(b) an electric pulse producing oscillator in said encapsulation,

(c) an electric battery in said encapsulation and connected with the oscillator for energizing it,

(d) a time constant control circuit including a capacitor means and discharge series resistors connected with said oscillator to govern the characteristics of the pulses therefrom,

(e) a reed switch having two stable states and two magnetically susceptible contacts one of which is con nected at a juncture intermediate two of said resistors and the other of which is in circuit at a juncture beyond one of said resistors to by-pass the same when the contacts are in a closed state,

(f)'a permanent biasing magnet adjacent the reed switch the magnet field of which biasing magnet passes along the path established by said contacts and is of insufficient field strength to attract the contacts together in the absence of an auxiliary magnetic field but is of sufficient strength to maintain the contacts in one stable state when the contacts have been acted on by said auxiliary magnetic field, and

(g) manually positionable auxiliary magnet means which may be located externally in aligned proximity with said biasing magnet to thereby supplement or nullify the biasing magnet field to transfer the reed switch to a difierent stable state.

References Cited by the Examiner UNITED STATES PATENTS 3,050,695 8/1962 Du Vall l2842l X 3,195,540 7/1965 Waller 128-422 3,198,195 8/1965 Chardack 128-419 3,241,556 3/1966 Zacouto 128421 OTHER REFERENCES Kuck et al.: I.E.E.E. Transactions on Biomedical Electronics, volume BME 10, No. 3, July 1963, pages 117- 1 19.

RICHARD A. GAUDET, Primary Examiner.

W. E. KAMM, Assistant Examiner. 

